The present invention generally relates to power supplies for vehicle accessories, and more particularly pertains to a power supply for use in supplying power to an electrochromic mirror.
Automotive power systems are migrating from 12 V in cars and 24 V in some trucks towards higher voltages such as 42 V to reduce wiring weight and provide more electrical power for systems such as electric power steering and heat pumps. Electrochromic mirrors are driven with low voltages at relatively high currents. When using a linear series regulated power supply in a 12 V system, the power dissipation in the mirror can be in the range of 4 to 5 W (350 mA load). Using the same design approach with a 42 V supply would result in a 15 W power dissipation. A power dissipation of 5 W is in itself fairly difficult to handle in a small object such as a rearview mirror. A 15 W power dissipation presents a serious problem. Some mirrors, such as those that include displays or even dual displays (see, for example, commonly assigned U.S. patent application Ser. No. 09/359,616), can draw in excess of 500 mA and present an even larger and greater challenge with respect to thermal dissipation.
While voltage converters are generally known that could conceivably be utilized to convert a 42 V supply to lower, more practical voltages, such switched power supplies generate significant levels of electromagnetic interference (EMI). The rearview mirror is in a particularly critical location due to its proximity to the vehicle's radio antenna and the lack of a ground plane. To reduce cost, rearview mirrors are typically placed in plastic housings, which provide little EMI shielding. Thus, using such a switched power supply in a rearview mirror would result in EMI levels that greatly exceed the maximum levels accepted by manufacturers.